Circuits and methods for photoplethysmographic sensors

ABSTRACT

Some embodiments relate to a device, method, and/or computer-readable medium storing processor-executable process steps for processing a photoplethysmographic (“PPG”) signal in a monitoring device that monitors a property of blood flow. In some embodiments, the processing includes obtaining a first digital signal representing a detected light signal having a non-pulsatile (e.g., DC) component and a pulsatile component (e.g., AC). An offset control signal is generated from an estimation of the non-pulsatile component and a second digital signal is generated after subtracting the offset control signal from the detected light signal and applying a gain to the subtracted signal. A reconstructed signal is generated that is calculated from the gain and one or more of (i) the first digital signal, and (ii) the second digital signal and the offset control signal.

BACKGROUND Field

The embodiments described below relate to the measurement of biometricdata. Some embodiments relate to photoplethysmographic sensors.

Description

Recent consumer interest in personal health has led to a variety ofpersonal health monitoring devices being offered on the market. Suchdevices, until recently, tended to be complicated to use and weretypically designed for use with one activity, e.g., bicycle tripcomputers.

Recently, personal health monitoring devices (also referred to herein as“biometric tracking” or “biometric monitoring” devices) have expanded totrack multiple metrics of the wearer. For example, Fitbit, Inc. producesa number of biometric tracking devices that can have a number offeatures and elements, such as displays, batteries, sensors, wirelesscommunications capabilities, power sources, and interface buttons, aswell as mechanisms for attaching these devices to a pocket or otherportion of clothing or to a body part of the wearer, packaged within asmall volume which are configured to track, among other things, steps,distance, calorie expenditure, active minutes, elevation (e.g., asmeasured in floors or stairs), speed, pace, and the like.

To track these metrics, these devices may collect, process and display alarge variety of data using a variety of sensors. One type of sensorused in some biometric tracking devices is a heart rate sensor. Theseheart rate sensors typically operate by emitting light into the skin ofthe user and then measuring the light reflected or diffused back afterthe emitted light interacts with the user's skin.

SUMMARY

Some embodiments relate to a device, method, and/or computer-readablemedium storing processor-executable process steps for processing aphotoplethysmographic (“PPG”) signal in a monitoring device thatmonitors a property of blood flow. In some embodiments, the processingincludes obtaining a first digital signal representing a detected lightsignal having a non-pulsatile (e.g., DC) component and a pulsatilecomponent (e.g., AC). An offset control signal is generated from anestimation of the non-pulsatile component and a second digital signal isgenerated after subtracting the offset control signal from the detectedlight signal and applying a gain to the subtracted signal. Areconstructed signal is generated that is calculated from the gain andone or more of (i) the first digital signal, and (ii) the second digitalsignal and the offset control signal.

In some embodiments, generation of the offset control signal includesselecting a value for the offset control signal such that the seconddigital signal, which is generated after the offset control signal issubtracted from the first digital signal and adjusted by the gain, doesnot exceed a range threshold. The offset control signal may be generatedby selecting an initial value, determining that the first digital signalsubtracted by the offset control signal and then adjusted by the gaindoes not exceed a range threshold, selecting a subsequent value for theoffset control signal, and determining that the first digital signaladjusted by the subsequent value of the offset control signal and gaindoes not exceed the range threshold.

In some embodiments, the monitoring device may include multiple lightemitters to generate a source light signal. A control unit associatedwith the monitoring device may switch between the light emitters togenerate the source light signal. The multiple light emitters mayinclude emitters that emit the source light in different wavelengthregions (e.g., such as red, green and infrared regions).

In some embodiments, a monitoring device may be provide which supportstwo modes of operation to measure different properties of a blood flow.For example, in some embodiments, the device may allow measurement ofboth heart rate and blood oxygenation levels. In some embodiments, amethod for processing a photoplethysmographic (PPG) signal in amonitoring device includes receiving a signal selecting a mode ofoperation of the monitoring device. A switching component is operated toselect a first light source in a first mode of operation and a secondlight source in a second mode of operation. A selected one of the firstand the second light sources are operated to generate a source lightsignal. A first digital representation of the detected light signal isobtained, and a second digital signal is generated based at least inpart on the selected mode of operation. The second digital signal isprovided to a processor for use in measuring a property of blood flow.In some embodiments, the first mode of operation is a mode to measure aheart rate and a second mode of operation is a mode to measure a bloodoxygenation level. In some embodiments, when a first mode of operationis selected, an offset control component is disabled, and when a secondmode of operation is selected, the offset control component is enabledto allow subtraction of a DC component of the detected light signal.

Embodiments as described herein provide a number of advantages. Forexample, embodiments provide monitoring devices with reduced cost whichcan efficiently extract and resolve the pulsatile component of a PPGsignal allowing improved processing of PPG signals. In some cases,embodiments provide such improved processing without use of highresolution analog-to-digital (“ADC”) converters or oversamplingtechniques which can be expensive and consume additional power. Further,embodiments allow faster processing and signal acquisition time.

A more complete understanding of some embodiments can be obtained byreferring to the following detailed description and to the drawingsappended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The construction and usage of embodiments will become readily apparentfrom consideration of the following specification as illustrated in theaccompanying drawings, in which like reference numerals designate likeparts, and wherein:

FIG. 1 is a top perspective view of a device according to someembodiments;

FIG. 2 is a bottom perspective view of a device according to someembodiments;

FIG. 3 is a block diagram of a device according to some embodiments;

FIG. 4 is a block diagram of a sensor pursuant to some embodiments;

FIG. 5 is a block diagram of components of a sensor pursuant to someembodiments;

FIGS. 6A-6C are diagrams illustrating a photoplethysmographic (“PPG”)signal as processed by the device of FIGS. 4 and 5 pursuant to someembodiments;

FIG. 7 is a diagram illustrating a PPG signal as processed by the deviceof FIGS. 4 and 5 pursuant to some embodiments; and

FIG. 8 is a diagram illustrating a reconstructed PPG signal pursuant tosome embodiments.

DETAILED DESCRIPTION

The following description is provided to enable any person in the art tomake and use the described embodiments. Various modifications, however,will remain readily apparent to those in the art. A specific examplewill now be described with reference to the appended figures in order toprovide an introduction to various features. Embodiments are not limitedto the features or description of this example.

As discussed above, monitoring devices utilizing the methods and/orcircuitry discussed herein can be used to collect and monitor biometricinformation. An example of such a monitoring device 100 is shown inFIG. 1. According to the illustrated embodiment, the device 100 iswearable on a user's wrist. The device 100 includes a display 140, whichmay comprise any suitable type of display screen, and which may displaygraphical indicators based on biometric and other data detected,collected, monitored or otherwise generated by the device 100. Thedevice 100 may include one or more buttons 180 which may be manipulatedby a user to provide input to the device 100. The display 140 may alsoincorporate one or more input devices (such as a touch screen). A band190 may be wrapped around the wrist and is securable using one or moresecuring elements 195 (e.g., hook and loop, clasp, shape memoryelements, magnets). The shape and configuration of the device 100 is oneexample configuration within which embodiments of the present inventionmay be deployed. The monitoring devices and PPG sensor system andmethods set forth herein may be used with desirable results in deviceshaving a wide variety of shapes and configurations, and the shape andconfiguration illustrated in FIG. 1 and FIG. 2 are for illustrativepurposes.

FIG. 2 is a bottom view of the device 100, showing a sensor component210 and a power interface 220. The sensor component 210 may includesensors which benefit from close proximity and/or contact with a user'sskin tissue. Such sensors may include PPG sensors (e.g., heart rate,pulse oximeter, and the like), moisture, temperature, and/or capacitivetouch sensors (e.g., to detect when the device is being worn). The powerinterface 220 may interface with a docking station or other power sourceto receive electrical charge for charging of batteries located withinthe device 100. Although a single sensor component 210 is shown forsimplicity, multiple sensor components may be provided. Further, whilethe sensor component 210 is illustrated in FIG. 2 as protruding somewhatfrom the device 100, other embodiments may place sensors in proximity tothe user without use of a distinct protrusion.

Features of a particular category or type of sensor—a “PPG” or“photoplethysmographic” sensor—will be described in further detailherein. PPG sensors, such as a heart rate monitor or pulse oximeter, usea light-based technology to sense pulsating blood flow as controlled bythe heart's pumping action. PPG sensors may be used to measure a user'sheart rate, blood oxygenation, and other biometric parameters. In thedevice 100 shown in FIG. 1 and FIG. 2, sensor component 210 may shieldor be associated with one or more light sources (e.g., such asphotodiodes, light emitting diodes or “LEDs”) and light detector(s) andcorresponding control circuitry (e.g., as described further below). Insome cases, light pipes may be used to optically connect the lightsource(s) or the detector with the surface of the user's skin tissue.Beneath the skin, the light from the light sources scatters off of bloodin the body, some of which may be scattered or reflected back into aphotodetector located behind the sensor component 210. In someembodiments, as will be described further herein, the sensor component210 may be shaped and formed to improve the operation of the sensor. Forexample, in some embodiments, the sensor component 210 may utilizelight-transmissive structures to improve the performance of a PPGsensor. For example, a light-transmissive structure may include a maskconsisting of an opaque material that limits the aperture of one, someor all of the light source(s) and/or detector(s). In this way, thelight-transmissive structures may selectively define or control apreferential volume of the user's body that light is emitted into and/ordetected from.

FIG. 3 is a block diagram of a monitoring system 300 according to someembodiments. System 300 may be operated to control the collection andusage of biometric data pursuant to some embodiments. In some cases, themonitoring system 300 may implement internal features of the device 100shown in FIGS. 1 and 2.

As FIG. 3 shows, the system 300 includes one or more processing units310 (e.g., hardware elements, such as processor cores and/or processingthreads, discrete or integrated logic, and/or one or more statemachines, and/or field programmable gate arrays (or combinationsthereof)). In some cases, one or more processing units 310 areconfigured to execute processor-executable program code to cause thedevice 300 to operate as described herein, and memory 320 for storingthe program code and any other suitable data. The memory 320 maycomprise one or more fixed disks, solid-state random access memory,and/or removable media (e.g., a thumb drive) mounted in a correspondinginterface (e.g., a USB port).

A display interface 330 provides communication with the display 340,which may comprise any system for visual presentation of informationthat is or becomes known. The display 340 may comprise a touch screenfor receiving user input into the system 300 according to someembodiments. The display 140 shown in FIG. 1 is an example of thedisplay 340.

One or more processing units 310 may execute processor-executableprogram code stored in memory 320 to cause the system 300 to processsensor data control the operation of sensors and related components, andto perform operations as discussed herein. In some embodiments, theprocessing unit 310 may be configured as a processor core, memory andprogrammable input and output peripherals and may be referred to hereinas a microcontroller (or “MCU”). According to some embodiments, thesystem 300 comprises an integrated device such as, but not limited to, awearable unit (e.g., around a user's wrist such as the device 100 shownin FIGS. 1 and 2, around the user's neck, attached to an earlobe orinserted in an ear (e.g., an earbud), on a ring, or the like) or anotherwise portable unit (e.g., a smartphone, a dedicated music player, afob). In some embodiments, elements of the system 300 may be embodied inseparate devices, such as a server device (e.g., a desktop computer)including elements 310, 320 and 330, and a terminal device (e.g., awatch) including the display 340. The system 300 may perform functionsother than those attributed thereto herein, and may include any elementsthat are necessary for the operation thereof.

Some embodiments of the system 300 include a portable monitoring devicehaving a physical size and shape adapted to couple to the body of auser, which allows the user to perform normal or typical user activities(including, for example, exercise of all kinds and type) withouthindering the user from performing such activities. An example of such adevice is the device 100 of FIG. 1. The portable monitoring device mayinclude a mechanism (for example, a clip, strap and/or tie) thatfacilitates coupling or affixing the device to the user during suchnormal or typical user activities.

The system 300 further includes one or more sensor interfaces 350 forexchanging data with one or more sensors 360. The sensors 360 maycomprise any sensors for acquiring data including biometric monitoringdata. Examples of sensors 360 include, but are not limited to, anaccelerometer, a light sensor, a compass, a switch, a pedometer, a bloodoxygen sensor, a gyroscope, a magnetometer, a Global Positioning Systemdevice, a proximity sensor, a pressure sensor (e.g., an altimeter), anda heart rate sensor. One or more of the sensors 360 may share commonhardware and/or software components.

As shown in FIG. 3, a user 370 is pictured to indicate that, accordingto some embodiments, the user 370 influences the data acquired by one ormore of the one or more sensors 360. For example, the one or moresensors 360 may generate data based on physical activity of the user370. Moreover, one or more of sensors 360 may generate data via directcontact with the user, for example during heart rate, skin temperature,and/or blood oxygen monitoring.

Reference is now made to FIG. 4, where an example block diagram of asensor 400 is shown which may be used as one of multiple sensors 360 ofthe device 300 in FIG. 3. As will be described further below,embodiments of the sensor 400 may be operated or configured to functionas both an optical heart rate sensor and a blood oxygenation sensor. Asshown in FIG. 4, the sensor 400 includes one or more light sources 410which emit light toward a user's skin tissue, and where the reflectionand/or diffusion of such light from the skin/internal tissues of theuser is sensed by one or more light detectors 420, the signal from whichis subsequently digitized by one or more analog to digital converters(“ADC”) 432, 434 (referred to as “ADC 1” and “ADC 2”, respectively).

In general, a PPG signal is an optically and non-invasively obtainedsignal used to sense volumic changes of blood vessels. A light source410 emits at the tissue of interest (e.g., the skin shown in FIG. 4) anda light detector 420 is placed to measure the transmitted or reflectedlight. Depending on the wavelengths of the light source 410, informationabout a person such as heart rate and blood oxygenation can bedetermined.

The light sources 410 may be, for example, one or more photodiodes whichare configured to generate a light in a wavelength. For example, aphotodiode may be provided as a light source 410 and configured togenerate light in the red, infrared, or green regions (660 nm, 940 nm,and 528 nm, respectively). As will be described further herein, thesensor 400 may be configured with one or more light sources 410operating in the infrared and red wavelengths for use in obtaining bloodoxygen measurements (e.g., operating as an SpO2 sensor). One or moreadditional light sources 410 may be provided which operate in the greenwavelength region for use as a heart rate sensor.

Embodiments of the present invention allow the sensor 400 to be operatedin different modes of operation depending on whether the infrared/redwavelengths or the green wavelengths are used. For example, the sensormay be operated in a mode to discern information about a user's heartrate (when the green wavelength light sources 410 are operated) and in amode to discern information about a user's blood oxygenation (when theinfrared and red wavelength light sources 410 are operated). In general,both the infrared and red PPG signals' pulsatile components are muchweaker in comparison to that of the green wavelength signals and aremore susceptible to uncertainty due to noise. In the infrared and redwavelengths, there is a large offset component of the PPG signal(referred to herein as the “non-pulsatile” or “DC” component) and thepulsatile component (also referred to herein as the “AC” component). Forexample, the AC component of the detected PPG signal can be as small as0.05% of the signal, leaving up to 99.95% of the dynamic range unusable.Put another way, in a sensor using 12-bit resolution ADCs, the besteffective resolution of the AC component of the PPG signal may be lessthan 2-bits. The use of very high-resolution ADCs to enable highereffective resolution of the AC component is undesirable in many sensorsystems, particularly where power, cost and size are important.

In order to obtain accurate signal information for a processor (such asthe processor 310 of FIG. 3) or microcontroller (shown as MCU 402) toperform processing of the detected signals, the light informationdetected by the light detectors 420 is processed using a front-endsignal conditioning block 430 which directly interfaces with the lightdetectors 420 and which outputs an analog PPG signal. This PPG signal isprovided to an ADC 432 which is configured with sufficient resolution tomeasure the non-pulsatile (DC) component of the PPG signal (but maybenot enough resolution for the pulsatile or AC component). This digitalsignal 433 (which may be referred to elsewhere herein as the “firstdigital signal”) is provided to the MCU 402 for processing. Upon receiptof the first digital Signal 433, the MCU 402 performs processing togenerate an offset signal (as will be described further herein). Theoffset signal is provided from the MCU 402 to an offset removal stage440. The offset removal stage 440 receives the offset signal from theMCU 402 and removes the offset value from the signal 530. The MCU 402calculates what value the offset signal should be and provides it to theoffset removal stage 440 to remove the value of the offset signal fromthe signal from ADC 432. In general, the offset removal stage 440provides a modified PPG signal representing primarily the pulsatile (orAC) component of the PPG signal to a second ADC 434 for digitization andfor provision to the MCU 402 for use in processing to determine adetected blood oxygenation level. In some embodiments, the MCU 402includes memory which stores control and application data as well assignal data. In some embodiments, the MCU 402 stores data such as thedigital signal 433, offset signal data and the signal received from ADC434 in a memory as time series data. For example, the signal data may bestored such that the MCU 402 may reconstruct a PPG signal as describedfurther below in conjunction with FIGS. 7 and 8.

As discussed above, pursuant to some embodiments, multiple light sources410 may be provided (e.g., including light sources for differentwavelengths). The operation and selection of different light sources (aswell as the intensity of the selected light source 410) may becontrolled using one or more light source controls 412. The light sourcecontrols 412 may be operated by control signals received from the MCU402. For example, in some embodiments, a light source 410 may becontrolled to enable the light source 410 (and, in some embodiments, toset a desired intensity) based on a mode of operation selected by theMCU 402. As one illustrative example, a mode of operation may be a modein which an optical heart rate is to be detected. In such a mode, one ormore light sources 410 capable of generating light in a green wavelengthregion may be operated. As another illustrative example, a mode ofoperation may be a mode in which a blood oxygenation measurement is tobe taken. In such a mode, one or more light sources 410 capable ofgenerating light in an infrared wavelength region may be operated aswell as one or more light sources 410 capable of generating light in ared wavelength region. In some embodiments, the MCU 402 may be operatedto switch between different light sources 410 and/or different lightdetectors 420 based on a desired mode of operation.

The light source control module 412 may be controlled by the MCU 402.The MCU 402 receives digital signals from one or both of the ADCs 432,434 and may use that information to generate control signals includingthe offset control signal as well as control signals to operate thelight source control 412 and light sources 410. The MCU 402 mayadaptively control the operation of the light source 412 based onoperating information received from the first and/or second ADCs (432,434), allowing a wide variety of operating controls. Further, in someembodiments, the MCU 402 may control whether the second ADC 434 isoperated based on the mode of operation (e.g., in some embodiments, thesecond ADC 434 is not operated when the sensor is in a mode of operationto obtain an optical heart rate measurement).

Features of some embodiments will now be described in further detail byreference to FIG. 5, which is a block diagram depicting components ofthe sensor device of FIG. 4 in further detail. More particularly, FIG. 5depicts the logical blocks and functions that allow the generation ofsignals for provision to the MCU 520 for further processing of bloodoxygenation analysis and heart rate analysis. As shown in FIG. 5, asensor device may be provided with a number of components, including oneor more photodiodes 502, 504 and one or more signal conditioningcomponents 506, 508 which correspond to the photodiodes. While two setsof photodiodes 502, 504 and signal conditioning components 506, 508 areshown, sensors may be provided with more than two sets as will bedescribed further herein. The sensor further includes or is incommunication with an MCU 520 that performs processing to controlcomponents of the sensor and to perform processing of signals generatedby the sensor. For example, the MCU 520 may generate control signals tooperate a multiplexor 510 which allows control over which photodiode502, 504 to use as the signal source. For example, the MCU 520 may issuecontrol signals to select photodiode 502 and signal conditioning block506 as the source signal.

As a more specific illustrative example, the MCU 520 may generate thecontrol signal based on a mode of operation selected by a user of themonitoring device (e.g., the user may select to operate the monitoringdevice to capture an optical heart rate measurement, or the user mayselect to operate the monitoring device to capture a blood oxygenmeasurement). For example, the photodiode 502 may be a photodiode thatoperates in a green wavelength region, and the photodiode 502 (andassociated signal conditioning component 506) may be enabled by acontrol signal from the MCU 520 which selects that signal source throughthe multiplexor 510. In the following discussion, an embodiment will bedescribed in which the photodiode 502 is a green wavelength photodiode,and the photodiode 504 is a set of photodiodes that operate in theinfrared and red wavelengths and are used for SpO2 measurements.

Once the MCU 520 has selected a PPG signal source (e.g., photodiode 502or photodiode 504), the multiplexor 510 may pass the PPG signal from theselected signal source to an ambient removal component 512. The PPGsignal is shown on FIG. 5 as signal S1(t). The signal S1(t) is alsoprovided as an input to the MCU 520 and is used for further processingas described below. The PPG signal S1(t) is processed by an ADC (such asthe stage one ADC 432 of FIG. 4). In general, the input to the MCU 520is a digital representation of the PPG signal S1(t) that represents thePPG signal (including both the AC and the DC components of the signal)at a point in time. A sample illustration of the PPG signal S1(t) isshown in FIG. 6A, where the magnitude of the PPG signal is shown overtime. As shown, the signal has a relatively small amount of information(shown as region 602) with a large offset (shown as region 604).

In some embodiments, the PPG signal S1(t) may further be processed usingan ambient removal component 512 which operates to remove an ambientcomponent of the signal (although this processing is optional and willnot be described in detail herein).

The MCU 520, having received the digital representation of the PPGsignal S1(t), performs processing to generate an offset signal. Theoffset signal may be selected or determined by the MCU 520 based on apredetermined value selected such that the gain times the result of thesignal S1(t) less the offset value is within the dynamic range of theADCs of the sensor. The offset value may be dynamically updated as S1(t)is received by the MCU 520. In some embodiments, the offset value may beselected such that the adjusted signal is above a predetermined orcalculated minimum, but also below a maximum (such that the signal S1(t)is within the dynamic range of the ADCs of the sensor). Referring toFIG. 6B, a sample illustration of the signal S1(t) less the offsetsignal is shown. The offset signal is provided as an input to a DCoffset signal buffer 516 for provision to a DC subtraction component514. The DC offset signal buffer 516 may include a digital to analogconverter (“DAC”) to generate the appropriate offset voltage level basedon the offset signal from the MCU 520. The DC subtraction component 514generates a second PPG signal S2(t) which is generally equivalent to again times the result of the first signal (S1(t)) less the DC offset.Put another way, the output of the DC subtraction component 514 isgenerally equivalent to the AC component of the PPG signal with some DCleft. A representation of the signal S2(t) is shown in FIG. 6C. Ingeneral, the offset (illustrated in FIG. 6B) is selected to allow thegain (applied in FIG. 6C) to be applied while staying within the dynamicrange of the system. The result is a signal S2(t) that has a larger ACcomponent. The signal S2(t) is provided to a second ADC (such as the ADC434 of FIG. 4) to generate a digital representation of the signal S2(t)for provision to the MCU 520.

Reference is now made to FIG. 7, where a representation of the PPGsignals as processed by the MCU 520 over time are shown. In the topportion of FIG. 7, the signal S1(t) over time is shown. As shown, theoffset determined by the MCU 520 may vary over time, but is selectedsuch that the signal S2(t) does not extend beyond the dynamic range ofthe ADCs of the system. The MCU 520 stores the signal information forS1(t), S2(t), the gain, and the offset(t), and this signal informationis associated with a time period for which it is relevant. The differenttime periods (with the offset values shown in the stage 1 ADC chart) areillustrated with vertical lines separating each time period. The MCU 520may then perform processing to reconstruct the PPG signal. Referring toFIG. 8, the MCU 520 may perform processing to construct a PPG signalS3(t) which may be calculated from S1(t), S2(t), the gain, and theoffset(t). In general, S3(t) is equivalent to S1(t) multiplied by thegain, or S2(t) plus the gain multiplied by the offset(t). The resultingsignal (S3(t)) allows the MCU 520 to perform processing on a signal withmore resolution and without need for expensive high-resolution ADCs oroversampling to resolve the AC component of the PPG signal. In thismanner, embodiments provide more resolution for PPG signals because theDC component of the PPG signal can be subtracted before amplification.

Further, embodiments allow a monitoring device to be operated withdifferent types of biometric measurements. For example, devicesconsistent with the embodiments shown in FIGS. 4 and 5 may be operatedto obtain measurements for both heart rate (which may use photodiodesoperating in the green wavelength) as well as SpO2 (which may usephotodiodes operating in the red and infrared wavelengths). Pursuant tosome embodiments, the DC subtraction component 514 operatesindependently of which mode of operation is used. In some embodiments,when the device is operated in a mode to obtain heart rate measurements,a photodiode 502 operating in the green wavelength is enabled (orselected as the PPG signal source by the multiplexor 510) and the offsetcontrol signal is set to equal zero. When the device is operated in amode to obtain SpO2 measurements, photodiodes 504 operating in the redand infrared wavelengths are enabled (or selected as the PPG signalsource by the multiplexor 510) and the offset control signal isdetermined as described above. Further, depending on the selected modeof operation, the MCU 520 may be enabled to reconstruct a signal S3(t)(as shown in FIG. 8) or the signal reconstruction may be disabled. Forexample, when the device is operated in a SpO2 measurement mode ofoperation the offset control signal is determined as described above andthe MCU 520 is configured to perform a signal reconstruction. When thedevice is operated in a heart rate measurement mode of operation, theoffset control signal is set to zero and the MCU 520 is configured tonot perform a signal reconstruction. In this manner, embodiments allowmultiple modes of operation using the same components, without expensivehigh-resolution ADCs to resolve the AC component of signals(particularly for SpO2 modes of operation).

The foregoing diagrams represent logical architectures for describingprocesses according to some embodiments, and actual implementations mayinclude more or different components arranged in other manners. Othertopologies may be used in conjunction with other embodiments. Moreover,each system described herein may be implemented by any number of devicesin communication via any number of other public and/or private networks.Two or more of such computing devices may be located remote from oneanother and may communicate with one another via any known manner ofnetwork(s) and/or a dedicated connection. Each device may include anynumber of hardware and/or software elements suitable to provide thefunctions described herein as well as any other functions. For example,any computing device used in an implementation of some embodiments mayinclude a processor to execute program code such that the computingdevice operates as described herein.

The modules and components described herein may be implemented inhardware, a processor executing firmware and/or software instructions,or any combination thereof. For instance, memory portions of processingmodules may be implemented in one or more memory devices such as one ormore of a magnetic disk, an optical disk, a random access memory (RAM),a video RAM, a Flash memory, etc. Similarly, processing units ofprocessing or control modules (such as, for example, the light sourcecontrol module 412 of FIG. 4) may be implemented using one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), a programmable logic device (PLD), etc. Ifthe module, processor and/or processing units are implemented using aprocessor executing firmware and/or software instructions, the softwareor firmware instructions may be stored in any computer readable memorysuch as on a magnetic disk, an optical disk, in a RAM or ROM or Flashmemory, a memory of a processor (e.g., a cache memory), etc. Theprocessor executing firmware and/or software instructions may comprise ageneral purpose processor or a special purpose processor such as adigital signal processor (DSP), a graphics processor, or the like.

Those in the art will appreciate that various adaptations andmodifications of the above-described embodiments can be configuredwithout departing from the scope and spirit of the claims. Therefore, itis to be understood that the claims may be practiced other than asspecifically described herein.

What is claimed is:
 1. A method for processing a photoplethysmographic(PPG) signal in a monitoring device that measures a property of bloodflow, the method comprising: causing one or more first light sources toemit light in the green wavelength spectrum during first periods oftime; causing one or more second light sources to collectively emitlight in the red and infrared wavelength spectra during second periodsof time different from the first periods of time; obtaining a detectedlight signal from one or more detectors during the first and secondperiods of time, the detected light signal including pulsatile andnon-pulsatile components; obtaining a first digital signal representingthe detected light signal; generating, from the first digital signal, anoffset control signal from an estimation of the non-pulsatile component;generating a second digital signal by subtracting the offset controlsignal from the detected light signal to produce a subtracted signal andthen applying a gain to the subtracted signal; generating areconstructed signal calculated from the gain, the second digitalsignal, and the offset control signal; setting the offset control signalto zero while generating the reconstructed signal for each first periodof time; outputting data derived from portions of the reconstructedsignal associated with the first periods of time as a heart rate; andoutputting data derived from portions of the reconstructed signalassociated with the second periods of time as a blood oxygenation level.2. The method of claim 1, wherein generating the offset control signalincludes: selecting a value for the offset control signal such that thesecond digital signal does not exceed a range threshold.
 3. The methodof claim 2, wherein generating the offset control signal includes:selecting an initial value for the offset control signal; determiningthat the second digital signal exceeds a range threshold; selecting asubsequent value for the offset control signal; and determining that thesecond digital signal does not exceed the range threshold.
 4. The methodof claim 1, further comprising: constructing a digital representationover time of the detected light signal based on the data derived fromthe reconstructed signals.
 5. The method of claim 4, wherein one or moreof the second periods of time are consecutive and at least one of theoffset control signals and the gains used to generate the reconstructedsignal for the one or more consecutive second periods of time variesfrom one second period of time to the next second period of time.
 6. Themethod of claim 1, wherein the first digital signal is generated by ananalog-to-digital converter with sufficient resolution to measure,during the second periods of time, the non-pulsatile component but notthe pulsatile component.
 7. The method of claim 1, wherein the detectedlight signal represents the light emitted by the one or more first lightsources or the one or more second light sources after interacting with askin tissue, wherein the method further comprises: operating a switchingcomponent to switch between the one or more first light sources and theone or more second light sources.
 8. The method of claim 1, furthercomprising: generating the detected light signal using one or more firstphotodetectors during the first periods of time, generating the detectedlight signal using one or more second photodetectors during the secondperiods of time, and operating a switching component to switch betweenthe one or more first photodetectors and the one or more secondphotodetectors.
 9. The method of claim 1, wherein the one or more firstlight sources are configured to emit light in a wavelength region ofapproximately 528 nm.
 10. The method of claim 1, wherein the one or moresecond light sources are collectively configured to emit light inwavelength regions of approximately 660 nm and approximately 940 nm. 11.The method of claim 1, wherein: the one or more first light sources areconfigured to emit light in a wavelength region of approximately 528 nm,the one or more second light sources are collectively configured to emitlight in wavelength regions of approximately 660 nm and approximately940 nm.
 12. The method of claim 7, wherein the operating the switchingcomponent to switch between the one or more first light sources and theone or more second light sources is based on a selected mode ofoperation of the monitoring device.
 13. The method of claim 1, furthercomprising: removing, from the first digital signal and prior togenerating the second digital signal, a signal corresponding to acomponent of the detected light signal that is not the result ofoperation of the one or more first light sources or the one or moresecond light sources.
 14. The method of claim 1, further comprisingstoring one or more of the first digital signal, the offset controlsignal, and the second digital signal in a memory in a time series dataset.
 15. The method of claim 8, further comprising: operating an emitterswitching component to switch between the one or more first lightsources and the one or more second light sources.
 16. Aphotoplethysmographic monitoring device, comprising: one or more firstlight sources configured to emit light in the green wavelength spectrum;one or more second light sources configured to collectively emit lightin the red and infrared wavelength spectra; one or more detectors; oneor more processors; and a non-transitory, computer-readable mediumstoring instructions which, when executed, cause the one or moreprocessors to: cause one or more first light sources to emit light inthe green wavelength spectrum during first periods of time; cause one ormore second light sources to collectively emit light in the red andinfrared wavelength spectra during second periods of time different fromthe first periods of time; cause a detected light signal to be obtainedfrom one or more detectors during the first and second periods of time,the detected light signal including pulsatile and non-pulsatilecomponents; cause a first digital signal representing the detected lightsignal to be obtained; cause an offset control signal to be generatedfrom an estimation of the non-pulsatile component in the first digitalsignal; cause a second digital signal to be generated by subtracting theoffset control signal from the first digital signal to produce asubtracted signal and then applying a gain to the subtracted signal;cause a reconstructed signal to be calculated from the gain, the seconddigital signal, and the offset control signal; cause the offset controlsignal to be set to zero while generating the reconstructed signal foreach first period of time; cause data derived from portions of thereconstructed signal associated with the first periods of time to beoutput as a heart rate; and cause data derived from portions of thereconstructed signal associated with the second periods of time to beoutput as a blood oxygenation level.
 17. The photoplethysmographicmonitoring device of claim 16, wherein the non-transitorycomputer-readable medium stores further instructions which, whenexecuted, cause the one or more processors to: operate a switchcomponent to switch between the one or more first light sources and theone or more second light sources.
 18. The photoplethysmographicmonitoring device of claim 16, wherein the one or more detectorsincludes one or more first photodetectors and one or more secondphotodetectors and wherein the non-transitory computer-readable mediumstores further instructions which, when executed, cause the one or moreprocessors to: generate the detected light signal using one or morefirst photodetectors during the first periods of time, and generate thedetected light signal using one or more second photodetectors during thesecond periods of time.
 19. The photoplethysmographic monitoring deviceof claim 16, wherein the one or more first light sources are configuredto emit light in a wavelength region of approximately 528 nm and the oneor more second light sources, collectively, are configured to emit lightin a second wavelength region of approximately 660 nm and a thirdwavelength region of approximately 940 nm.
 20. The photoplethysmographicmonitoring device of claim 16, wherein the non-transitorycomputer-readable medium stores further instructions which, whenexecuted, cause the one or more processors to generate the offsetcontrol signal by selecting a value for the offset control signal suchthat the second digital signal does not exceed a range threshold. 21.The photoplethysmographic monitoring device of claim 20, wherein thenon-transitory computer-readable medium stores further instructionswhich, when executed, cause the one or more processors to: select aninitial value for the offset control signal; determine that the seconddigital signal exceeds a range threshold; select a subsequent value forthe offset control signal; and determine that the second digital signaldoes not exceed the range threshold.
 22. The photoplethysmographicmonitoring device of claim 16, wherein the non-transitorycomputer-readable medium stores further instructions which, whenexecuted, cause the one or more processors to: construct a digitalrepresentation over time of the detected light signal based on the dataderived from the reconstructed signals.
 23. The photoplethysmographicmonitoring device of claim 22, wherein one or more of the second periodsof time are consecutive and at least one of the offset control signalsand the gains used to generate the reconstructed signal for the one ormore consecutive second periods of time varies from one second period oftime to the next second period of time.
 24. The photoplethysmographicmonitoring device of claim 16, further comprising an analog-to-digitalconverter with sufficient resolution to measure, during the secondperiods of time, the non-pulsatile component but not the pulsatilecomponent, wherein the analog-to-digital converter is configured togenerate the first digital signal.
 25. A non-transitorycomputer-readable medium storing computer-executable instructions forprocessing a photoplethysmographic signal in a monitoring device thatmeasures a property of a blood flow, the computer-executableinstructions, when executed, causing one or more processors to: causeone or more first light sources to emit light in the green wavelengthspectrum during first periods of time; cause one or more second lightsources to collectively emit light in the red and infrared wavelengthspectra during second periods of time different from the first periodsof time; cause a detected light signal to be obtained from one or moredetectors during the first and second periods of time, the detectedlight signal including pulsatile and non-pulsatile components; cause afirst digital signal representing the detected light signal to beobtained; cause an offset control signal to be generated from anestimation of the non-pulsatile component in the first digital signal;cause a second digital signal to be generated by subtracting the offsetcontrol signal from the first digital signal to produce a subtractedsignal and then applying a gain to the subtracted signal; cause areconstructed signal to be calculated from the gain, the second digitalsignal, and the offset control signal; cause the offset control signalto be set to zero while generating the reconstructed signal for eachfirst period of time; cause data derived from portions of thereconstructed signal associated with the first periods of time to beoutput as a heart rate; and cause data derived from portions of thereconstructed signal associated with the second periods of time to beoutput as a blood oxygenation level.
 26. The non-transitorycomputer-readable medium of claim 25, further storingcomputer-executable instructions that, when executed, further cause theone or more processors to: select a value for the offset control signalsuch that the second digital signal does not exceed a range threshold.27. The non-transitory computer-readable medium of claim 26, furtherstoring computer-executable instructions that, when executed, furthercause the one or more processors to: select an initial value for theoffset control signal; determine that the second digital signal exceedsa range threshold; select a subsequent value for the offset controlsignal; and determine that the second digital signal does not exceed therange threshold.
 28. The non-transitory computer-readable medium ofclaim 25, further storing computer-executable instructions that, whenexecuted, further cause the one or more processors to: construct adigital representation over time of the detected light signal based onthe data derived from the reconstructed signals.
 29. The non-transitorycomputer-readable medium of claim 28, wherein: one or more of the secondperiods of time are consecutive and at least one of the offset controlsignals and the gains used to generate the reconstructed signal for theone or more consecutive second periods of time varies from one secondperiod of time to the next second period of time.